YES! GOT IT!
...
1) made backup with rupor's method (tool: Roadkil's Disk Image)
2) connected to pc in recovery mode
3) Used MiniTool Partition Wizard Home Edition 7.6.1 (from here: http://www.partitionwizard.com/free-...n-manager.html), pressing Apply button after EACH modification.
4) shrink READER partition (keep it at the end of the drive, creating free space between READER and /system)
5) move /system to the end of extended partition. Now there is free space between /data and /system)
6) tool cant enlarge /data to the beginning of /system, which is what i wanted to do. So i deleted /data and created new ext4 partition in the space that is now free. Labeled it "data". Then FORMATED it as "ext4"
7) Now /data is empty. I tried to reboot...

... and that's it , worked on first try without a glitch (still have the physical disk full backup just in case), thank you for the reference to MiniTool Partition Wizard, made that repartitioning a breeze under Windows 7, really minimal operations. Just slightly simplified your directives to reflect what I really did, because I didn't had any bad surprise, reader restarted with initial configuration on reboot and that's it. Perhaps because I applied changes step-by-step ? to you, uboot, rupor and porkupan